Theoretical Insight into Cp*Ir-Catalyzed Upgrading of Ethanol to n-Butanol: The Crucial Role of Cs2CO3

Abstract

The ethanol-to-n-butanol upgrading process catalyzed by a Cp*Ir complex (1-Ir) and mild base Cs₂CO₃ was investigated using density functional theory calculations. Initially, Cs₂CO₃ and 1-Ir form an active species 2 with an exothermicity of 12.9 kcal/mol. Ethanol dehydrogenation then occurs through the cooperation of the Ir center and Cs₂CO₃ to produce an Ir–H complex 3 with the release of acetaldehyde and CsHCO₃. Cs₂CO₃ catalyzes the aldol condensation of acetaldehyde to produce a C₄ intermediate crotonaldehyde. Subsequently, the successive hydride and proton migrations occur from 3 and ethanol to crotonaldehyde, respectively, to produce butanal. The proton migration step is the rate-determining step (ΔG^‡/ΔG = 29.1/–12.1 kcal/mol). Finally, n-butanol is produced via transfer hydrogenation of butanal with ethanol catalyzed by Cs₂CO₃. High selectivity for n-butanol is due to preferential hydrogenation of crotonaldehyde over its further condensation into C₆ species. Cs₂CO₃ plays a critical role in promoting ethanol dehydrogenation, aldol condensation, and butanal hydrogenation. In contrast, Na₂CO₃ significantly reduces reaction efficiency mainly due to its weaker basicity in the aldol condensation of acetaldehyde. These findings provide insights into the ethanol-to-n-butanol conversion and offer a foundation for developing milder bases for the reaction.